Militants from the al-Qaida-inspired Islamic State of Iraq and Syria (ISIS) celebrating the group's declaration of an Islamic state, in Fallujah, 40 miles (65 kilometers) west of Baghdad, Iraq, June 30, 2014.

IRBIL, Iraq — Iraq appeared closer to its long-predicted disintegration Thursday as Kurdish leaders ordered steps to hold a referendum as soon as possible on self-rule for their oil-rich territory and Islamist extremists improved their military position by seizing additional territory across the border in Syria.

U.S. military officials, offering their first public assessment of the situation, said it will take decades to subdue the threat now posed by the Islamic State, which seized Iraqs second largest city, Mosul, on June 10, then raced southward 200 miles, coming to within an hours drive of Baghdad before their advance was halted. On Sunday, it declared an Islamic caliphate on the land it controls in Syria and Iraq.

Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told reporters in Washington that the Islamic States rapid advance had stretched its resources. Still, he said he believed that the world community would have to wrestle with the organization for a generation or two.

He also labeled as bleak the future for a united Iraq unless there is reconciliation between Sunni Arabs, who have supported the Islamic States advances, and majority Shiite Arabs, who run the countrys government. Dempsey said Iraqs political leaders, an apparent reference to embattled Shiite Prime Minister Nouri al Maliki, have to find a way to separate Sunnis from the Islamic State, with whom theyve partnered because they have zero confidence in the ability of Iraqs politicians to govern.

He said the United States first task is to determine whether we have a reliable Iraqi partner that is committed to growing their country into something that all Iraqis will be willing to participate in.

If the answer to that is no, then the future is pretty bleak, Dempsey said.

The impact of the Islamic States success in Iraq became clear Thursday in Syria, where Islamist fighters took over five towns in the Deir el Zour region without a fight. The development allowed the Islamic State to secure its rear flanks, where it had been battling other Syrian rebel groups, and gain control over much of Syrias oil and natural gas production _ an economic boon for an organization that already has seen its coffers filled with booty captured in Iraq.

The Islamic State was able to make its startling advance in Syria after the Nusra Front, al Qaidas official Syrian affiliate and a bulwark of the movement to topple the government of President Bashar Assad, abandoned its positions in Deir el Zour, the capital of the province by the same name, and Abu Kamal, another Syrian city, according to the Syrian Observatory for Human Rights, a Britain-based group that monitors violence in Syria.

The observatory said that the withdrawal came after powerful Sunni tribes that previously had supported Nusra endorsed the caliphate that the Islamic State had declared over the weekend. Nusra has fought a bitter battle with the Islamic State over tactics and strategy in Syria.

Unconfirmed but credible reports in local media accounts also said that multiple units previously aligned or directly controlled by Nusra had defected and sworn allegiance to the Islamic State and its new caliphate.

Videos posted on the Internet said that tribes in the towns of Mayadeen, al Ashara and Burqas, all longtime allies of Nusra, had sworn allegiance to the Islamic State. A tribal leader in one of the videos even referred to the leader of the Islamic State, Abu Bakr al-Baghdadi, as Caliph Ibrahim, the name he was given in the Islamic States announcement of the new caliphate.

The clans of the city of Ishara, and the villages around it . . . and all of the factions in these areas . . . announce before God that they will cease fighting with the Islamic State, the tribal leader said.

An anti-Assad activist in Deir el Zour, Abu Abdulla, said that the Islamic States access to heavy weapons it seized in Iraq had been key to local officials decision to quickly pledge allegiance to the group. He said that in one town, Shehail, local officials on Wednesday met with rebel groups, including Nusra, and agreed to surrender.

They decided to avoid more bloodshed in what would have been a losing battle, he said in an interview conducted by Skype.

The fall of Shehail had a domino effect, and within 24 hours, four other towns surrendered to the Islamic State. Rami Abdurrahamn, the Syrian Observatorys director, reported on Facebook that some Nusra units remained engaged in combat in the area but appeared to be on the brink of collapse because of defections and withdrawals.

But it was what took place in Irbil on Thursday that was most likely to cement Iraqs eventual partition into regions along ethnic and sectarian lines.

Massoud Barzani, the president of the Kurdistan Regional Government, buoyed by Israeli Prime Minister Benjamin Netanyahus public support for independence, took the first legal steps Thursday toward holding a referendum on self-rule, instructing the regional Parliament to pass a bill setting up an election commission as soon as possible.

We have international support for independence, and those who do not support us do not oppose us, he said, according to an official statement.

Barzani also announced that Kurdish military forces would not withdraw from Kirkuk and other disputed areas outside Kurdistans borders that it occupied following the collapse of the Iraqi army.

It is time to decide about our self-determination and not to wait for other people to decide about us, he said, according to an audio of the closed-door remarks obtained by Associated Press.

The White House did not criticize Kurdish moves toward independence but reiterated its longstanding position that only a united government could counter the Islamic State. The best way for Iraq to confront the threat posed by the Islamic extremists is to unify the country, White House spokesman Josh Earnest said.

Were confident that Iraq can meet the threat, Earnest said. He added a disclaimer, however: But were confident that they wont be able to meet the threat if they dont act quickly to . . . pursue an inclusive agenda.

There was no immediate reaction from the central government in Baghdad to Barzanis call, though Maliki has accused the Kurds in recent days of using the Islamic States advance to pursue their own political agenda. On Wednesday, he said there were no provisions in Iraqs constitution for the vote the Kurds were contemplating.

McClatchy special correspondent Prothero reported from Irbil, McClatchy special correspondent Alhamadee from Istanbul. Contributing to this report were Nancy A. Youssef and Lesley Clark in Washington and Roy Gutman in Istanbul.

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